How does an object with no velocity have force

[nhmllr]

I'm sure many many people ask this question
If you're standing on the ground, you feel yourself being pushed into the ground (the weight on your feet)
However, your velocity is always zero
F= ∆p / ∆t
F= m∆v / ∆t
F= m(v_f-v_i) / ∆t
v_f = v_i = 0
F = m*0 / ∆t
F = 0
Then why do you feel a force!?

 

[timthereaper]

Well, just cause the velocity is zero doesn't mean there isn't a force. Your weight is being countered by the normal force on the ground. Now, there is no *net* force on you, so there is no momentum change. The equation you have there would describe the net force.

 

[nhmllr]

Well, just cause the velocity is zero doesn't mean there isn't a force. Your weight is being countered by the normal force on the ground. Now, there is no *net* force on you, so there is no momentum change. The equation you have there would describe the net force.

But then why do I feel a force?
What equation would describe it?

 

[rcgldr]

You feel non-moving forces via compressions or tensions within your body, or at the point of contact between internal and external objects.

 

[timthereaper]

You feel a force because gravity is accelerating you downward, creating a force. However, the Earth is exerting a force on you in the opposite direction, causing you not to sink to the center of the Earth. There are forces acting on you, but they are equal and opposite. Newton's Second Law (F=ma) describes net forces and since there is a zero net force, the velocity is zero. Newton's First Law says that an object maintains constant velocity unless acted on by external *unbalanced* forces. In the you vs. Earth case, there is a balanced force, so the velocity is constant (zero).

For net force, the simple static equation would be the sum of all forces in the vertical direction: F_you - N = 0. If the right hand side of the equation weren't zero, you would be moving up or down, depending on the negative or positive sign of the term.

I guess you could also argue that the mass of your body is being accelerated downward and puts your joints and things into a state of compression, making neurons fire and giving you the impression you're feeling a force.

 

[Drakkith]

This is called Equilibrium. From wikipedia's article on Force:

Equilibrium occurs when the resultant force acting on a point particle is zero (that is, the vector sum of all forces is zero).

 

[russ_watters]

But then why do I feel a force?
What equation would describe it?

F=F?

The equations you listed relate forces and motion and don't really have anything to do with how a force is generated. There are lots of ways to generate a force.

For example:
With a spring: f=-kx.
With a hydraulic cylinder/pump: f=a*p
With a lever: f=t/d

 

[Curl]

But then why do I feel a force?
What equation would describe it?

Because the nerves in your feet are compressed and it sends signals to your brain that they are so.

You need to learn to think instead of looking for equations you can plug numbers into.

 

[Kyle Stanley]

As for the "Why?" the fact is that weight is a force.

As for the "How?" the fact that the acceleration of gravity is constant on Earth at g.

From what I can see, you seem to understand that indeed a force is experienced and you are struggling with the concept of weight vs. mass.

Mass is constant no matter where you are in the universe, whereas weight varies due to the acceleration of gravity where the aforementioned mass exists.

If there does not exist any gravitational acceleration or motion then it is safe to say that in this most unusual situation that a force also does not exist. However, if you have a mass of a certain amount and you are in a location where the gravity is also a certain amount some force will exist as according to F=ma.

I hope this helps some.

 

[caspernorth]

The feeling can happen when you move any part of your body [ meaning you make a slight motion that opposes the force of gravity] or even when you stand still perfectly [ this case also is impossible as blood circulation by heart opposes gravity] you will feel this opposing force by your own body because your weight is acting on your legs and thus its compressed by a bit sending nerve signals to your brain giving the feeling.

 

[caspernorth]

The net force being zero means you shouldn't feel it. But one feel the force of gravity while making movements. even the slightest of movement in muscles require energy to do work against this force. This is why feel it.
Another way to describe this : force should create a change in momentum. so net force if present will make you move. this is why when you jump you feel the net force, because air friction isn't enough to hold you from falling. gravity wins. but a particle suspended in air won't feel it. for it the net force is zero.

this is also evident in the top of your jumping position. there the velocity is zero but your acceleration isn't.